1. Field of the Invention
The present invention relates to a shock absorbing type steering apparatus of an automobile, especially a shock absorbing type steering column assembly which is incorporated in the steering apparatus and utilized to transmit the movement of a steering wheel to a steering gear. The present invention also relates to a method of adjusting a contraction load of a shock absorbing type steering column assembly. The method is used to regulate a load required for contracting the total length of the steering shaft during a collision to a desired value.
2. Related Background Art
In a steering apparatus for an automobile, a steering mechanism is used to transmit the movement of a steering wheel to a steering gear. A first steering shaft having a steering wheel fixed to the upper end portion thereof is rotatably inserted in a steering column. This steering column is fixed to the lower surface of an instrument panel by upper and lower brackets. The upper end portion of a second steering shaft is connected through a first universal joint to a lower end portion of the first steering shaft which protrudes from a lower end opening of the steering column. Further, the lower end portion of this second steering shaft is connected through a second universal joint to a third steering shaft leading to a steering gear. In the steering mechanism thus constructed, the movement of the steering wheel is transmitted to the steering gear through the first steering shaft inserted through the steering column, the first universal joint, the second steering shaft, the second universal joint, and the third steering shaft to give a steering angle to wheels.
In the steering mechanism thus constructed, the steering column and the steering shafts are usually made into a shock absorbing type in which the total length shortens due to a shock in order to protect a driver during a collision. The structure described in Japanese Patent Application Laid-Open No. 8-91230 is known as such a shock absorbing type steering shaft. FIGS. 10 to 16 show the shock absorbing type steering shaft described in this application, while FIG. 17 to 21 show a method of manufacturing the shock absorbing type steering shaft which is also described in this application.
This shock absorbing type steering shaft 11 is constructed such that an outer shaft 12 and an inner shaft 13 are combined for relative displacement in an axial direction (the left to right direction as viewed in FIG. 10), whereby the total length of the shaft shortens when an impact force in the axial direction is applied. The outer shaft 12 as a whole is of a tubular shape and one end portion (the left end portion as viewed in FIGS. 10 and 14) thereof is subjected to drawing, whereby a small-diametered portion 14 is formed in this end portion. A female serration 15 is formed on the inner peripheral surface of this small-diametered portion 14. The inner shaft 13 as a whole is also of a tubular shape and one end portion (the right end portion as viewed in FIGS. 10 and 11) thereof is widened to thereby form a large-diametered portion 16. A male serration 17 is formed on the outer peripheral surface of this large-diametered portion 16 to be engaged with the female serration 15.
Also, the fore end portion (the right end portion as viewed in FIGS. 10 and 11) of the large-diametered portion 16 is squeezed a little in the diametral direction thereof, whereby a first deformed portion 18 of an elliptical cross-sectional shape is formed over a length L. The major axis d1 of this first deformed portion 18 is larger than the diameter d0 of the body portion of the large-diametered portion 16, and the minor axis d2 of the first deformed portion is smaller than this diameter d0 (d1&gt;d0&gt;d2). Note that the diameters of the large-diametered portion 16 on which the male serration 17 is formed are all represented by the diameter (pcd) of that portion of the serration which corresponds to a pitch circle.
On the other hand, the fore end portion (the left end portion as viewed in FIGS. 10 and 14) of the small-diametered portion 14 is also squeezed a little in the diametral direction thereof, whereby a second deformed portion 19 of an elliptical cross-sectional shape is formed over the length L. The major axis D1 of this first deformed portion 19 is larger than the diameter D0 of the body portion of the small-diametered portion 14, and the minor axis D2 of the second deformed portion 19 is smaller than this diameter D0 (D1&gt;D0&gt;D2). The diameters of the small-diametered portion 14 on which the female serration 15 is formed are also all represented by the diameter (pcd) of that portion of the serration which corresponds to a pitch circle.
The diameter D0 of the small-diametered portion 14 is made slightly larger than the diameter d0 of the large-diametered portion 16 (D0&gt;d0) so that the female serration 15 and the male serration 17 may be brought into loose engagement with each other in portions other than the first and second deformed portions 18 and 19. However, the major axis d1 of the first deformed portion 18 is made slightly larger than the diameter D0 of the body portion of the small-diametered portion 14 (d1&gt;D0) and the minor axis D2 of the second deformed portion 19 is made slightly smaller than the diameter d0 of the body portion of the large-diametered portion 16 (D2&lt;d0).
The outer shaft 12 and the inner shaft 13 having such shapes as described above are combined together as shown in FIG. 10 to thereby provide the shock absorbing type steering shaft 11. More specifically, the large-diametered portion 16 formed on one end portion of the inner shaft 13 is located inside the small-diametered portion 14 formed on one end portion of the outer shaft 12, and the female serration 15 on the inner peripheral surface of the small-diametered portion 14 and the male serration 17 on the outer peripheral surface of the large-diametered portion 16 are brought into engagement with each other. In this state, the first deformed portion 18 formed on the fore end portion of the large-diametered portion 16 is pushed into a base end portion (the right end portion as viewed in FIGS. 10 and 14) of the small-diametered portion 14 while being elastically deformed (or plastically deformed). Also, the second deformed portion 19 formed on the fore end portion of the small-diametered portion 14 is pushed into a base end portion (the left end portion as viewed in FIGS. 10 and 11) of the large-diametered portion 16 while also being elastically deformed (or plastically deformed).
Accordingly, in the state in which the outer shaft 12 and the inner shaft 13 are combined together as shown in FIG. 10, the outer peripheral surface of the first deformed portion 18 is frictionally engaged with the inner peripheral portion of the base end portion of the small-diametered portion 14, and the inner peripheral surface of the second deformed portion 19 is frictionally engaged with the outer peripheral portion of the base end portion of the large-diametered portion 16, respectively. As a result, the outer shaft 12 and the inner shaft 13 are coupled together for the transmission of a rotational force between the two shafts 12 and 13, but against relative displacement in the axial direction so long as a strong force is not applied.
As described, the coupling between the outer shaft 12 and the inner shaft 13 is effected by bringing the first and second deformed portions 18 and 19 formed on the metallic outer shaft 12 and the inner shaft 13 into pressure-fitting with partner members and therefore, the heat resisting property of the coupling portion becomes sufficient and it never happens that the supporting force of the coupling portion becomes deficient depending on use conditions. Also, the first and second deformed portions 18 and 19 are provided at two axially spaced locations in the coupling portion between the outer shaft 12 and the inner shaft 13 and therefore, the bending rigidity of the coupling portion between the outer shaft 12 and the inner shaft 13 is also sufficiently secured.
Further, when a strong force is applied in the axial direction during collision, the outer shaft 12 and the inner shaft 13 are displaced relative to each other in the axial direction against a frictional force which is exerted on the pressure-fitted portions by the first and second deformed portions 18 and 19, to thereby shorten the total length of the shock absorbing type steering shaft 11. In the case of such a shock absorbing type steering shaft 11, the force required to shorten the total length suffices if it overcomes the frictional force exerted on the above-described two pressure-fitted portions. Accordingly, a contraction load (a collapse load) required to shorten the total length of the shock absorbing type steering shaft 11 is stable without becoming great, thereby effectively preventing a great impact force from being applied to a driver's body which has collided against the steering wheel in the course of an accident.
When the outer shaft 12 and the inner shaft 13 are to be combined together to thereby construct such a shock absorbing type steering shaft 11 as shown in FIG. 10, the two shafts 12 and 13 are first combined together as shown in FIG. 17. More specifically, the female serration 15 and the male serration 17 are brought into engagement with each other by the fore end portion of the small-diametered portion 14 and the fore end portion of the large-diametered portion 16. Then, with these serrations 15 and 17 kept in engagement with each other, the outer peripheral surface of the small-diametered portion 14 is pressed inwardly in the diametral direction thereof. That is, a pair of pressing pieces 20 and 20 are disposed around the fore end portion of the small-diametered portion 14 and the fore end portion of the large-diametered portion 16, and the pair of pressing pieces 20 and 20 are brought close to each other to thereby press the outer peripheral surface of the small-diametered portion 14 strongly. The inner side surfaces of these pressing pieces 20 and 20 which bear against the outer peripheral surface of the small-diametered portion 14 are provided with recesses 21 and 21 of an arcuate cross-sectional shape which are in close contact with this outer peripheral surface.
Gaps 22 and 22 having a thickness dimension .delta. are formed between the end surfaces of the pair of pressing pieces 20 and 20 with the recesses 21 and 21 brought into light contact with the outer peripheral surface of the small-diametered portion 14. Also, these pressing pieces 20 and 20 are strongly pressed toward each other by a pressing device, not shown, such as a hydraulic mechanism. So, if as shown in FIG. 19, the pair of pressing pieces 20 and 20 are moved toward each other until the thickness of the gaps 22 and 22 becomes zero, the cross-sectional shape of the fore end portion of the small-diametered portion 14 will be plastically deformed into an elliptical shape, as shown in FIG. 19. Further, the fore end portion of the large-diametered portion 16 which is inserted in the fore end portion of this small-diametered portion 14 is also pushed in the same direction through the two serrations 15 and 17. Then, the cross-sectional shape of the fore end portion of this large-diametered portion is also plastically deformed into an elliptical shape, as shown in FIG. 19.
In this manner, the fore end portion of the small-diametered portion 14 and the fore end portion of the large-diametered portion 16 are pressed inwardly in the diametral direction thereof and the cross-sectional shapes of these two fore end portions are plastically deformed into an elliptical shape, whereafter the outer shaft 12 and the inner shaft 13 are displaced relative to each other toward each other in the axial direction. That is, after these two shafts 12 and 13 have been taken out of the pair of pressing pieces 20 and 20, the outer shaft 12 is displaced leftward as viewed in FIG. 17 relative to the inner shaft 13 while the inner shaft 13 is displaced rightwardly as viewed in FIG. 17 relative to the outer shaft 12. Then, as shown in FIG. 10, the fore end portion of the small-diametered portion 14 is pressure-fitted onto the base end portion of the large-diametered portion 16, while the fore end portion of the large-diametered portion 16 is pressure-fitted into the base end portion of the small-diametered portion 14. The intermediate portion of the small-diametered portion 14 and the intermediate portion of the large-diametered portion 16 which are not plastically deformed by the pressing pieces 20 and 20 are brought into loose engagement with each other.
In the case of the structure described in the above-mentioned Japanese Patent Application Laid-Open No. 8-91230, as shown in FIG. 20, the inner surfaces of pressing pieces 20a and 20a for plastically deforming the fore end portions (see FIG. 17) of the small-diametered portion and the large-diametered portion which are engaged with each other are not formed with the recesses 21 and 21 (FIGS. 18 and 19), but are made into simple flat surfaces. Or, as shown in FIG. 21, a pair of pressing pieces 20b and 20b are formed into a V block shape so that the pressing pieces 20b and 20b press the fore end portions (see FIG. 17) of the small-diametered portion 14 and the large-diametered portion 16 which are engaged with each other at two locations each, i.e., four locations in total.
In the case of the shock absorbing type steering shaft 11 which is actually incorporated in a vehicle, a load necessary for shortening the total length is required to be regulated to a desirable value. If a so-called secondary collision occurs wherein the body of a driver collides with the steering wheel (FIG. 8) in the course of an accident, this steering wheel 1 is displaced forward while contracting the shock absorbing type steering shaft 11 and the steering column 3 (FIG. 8). In order to smoothly effect such forward displacement of the steering wheel 1 while absorbing the impact applied on the body of the driver, it is necessary to regulate the load required for contracting the shock absorbing type steering shaft 11 and the steering column 3 to a desirable value.
Also, it is necessary to provide a high bending rigidity of the steering shaft 11 and steering column 3.
In the Japanese patent application Laid-Open No. 8-91230 as described above, the pressing pieces 20, 20; 20a, 20a and 20b, 20b are pressed by a constant amount or length of the gaps .delta..
According to this technique, however, contraction load obtained somewhat fluctuates and also the bending rigidity fluctuates or is not stable.
Taking these circumstances into consideration, experiments were conducted by the method of the prior invention shown in FIGS. 17 to 21 to investigate the regulation of the contraction load. As a result, it was found that a difference in shape among the pressing pieces 20, 20a and 20b gives a large influence on the obtained contraction load of the shock absorbing type steering shaft.